Numerous automatic pipettes are known in the art and may be grouped into various categories. For example, some automatic pipettes employ positive displacement of a plunger that moves within a preformed tip to aspirate a desired volume, while other pipettes employ a pump (air interface pipette) that provides a reduced pressure that is sufficient for aspiration of the desired volume.
Positive displacement pipettes generally provide numerous advantages. For example, cross contamination between different samples aspirated by the same pipette (e.g., via aerosol) is virtually eliminated. Moreover, positive displacement pipettes often provide significantly higher accuracy and precision that is typically not achieved with air-interface pipettes, especially where high-vapor pressure liquids, detergent containing fluids, and/or volatile solvents are aspirated. However, such pipettes generally require close-tolerance manufactured pipette tips, which are relatively expensive. Moreover, disposable close-tolerance manufactured pipette tips for positive displacement pipettes are typically not available for use in an robotic pipettors. Alternatively, non-disposable pipette tips (e.g., Teflon coated tips) for positive displacement pipettes may be used in robotic pipettors. However, cross contamination frequently occurs unless specific additional cleaning steps are implemented into a robotic pipetting operation.
Air interface pipettes advantageously combine relatively inexpensive operation with satisfactory accuracy and precision for many applications. For example, where in a robotic air interface pipettor non-disposable tips are employed, a single vacuum pump may control multiple pipette channels, which decreases operating cost due to reduced maintenance of moving parts. However, cross contamination may arise, especially where no additional cleaning steps are implemented. To circumvent some of the problems with cross contamination, disposable pipette tips may be employed. However, and especially in robotic pipettors with multi-channel operation, disposal of used pipette tips may become problematic where the pipettor is enclosed within an analytic device.
Regardless of the type of pipette (i.e., air interface or positive displacement), further difficulties arise with the use of disposable pipette tips. Most significantly, where disposable pipette tips are engaged with (i.e., picked up by) a pipettor via a robotic mechanism, even relatively slight misalignment between the pipette tip and the pipettor will often result in a gap through which air is aspirated during a pipetting operation. Consequently, the volume of fluid in the pipette tip is lower than the volume that was intended to be aspirated. Moreover, since disposable pipette tips often have a relatively large manufacturing tolerance, the exact position of the tip of a pipette tip is difficult to predict. This becomes particularly problematic where surface tension of microliter and sub-microliter volumes requires close approximation of the tip to the surface where the volume is to be deposited.
Thus, although various systems for pipettes are known in the art, numerous problems still remain. Therefore, there is still a need for an improved methods and systems for automated pipettes, and especially those that employ disposable pipettes.